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On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the bulge
We present values for angular diameter, flux and extinction for 70Galactic planetary nebulae observed using narrow-band filters. Angulardiameters are derived using constant emissivity shell andphotoionization line emission models. The mean of the results from thesetwo models are presented as our best estimate. Contour plots of 36 fullyresolved objects are included and the low-intensity contours oftenreveal an elliptical structure that is not always apparent from fullwidth at half maximum measurements. Flux densities are determined, andfor both Hα and [OIII] there is little evidence of any systematicdifferences between observed and catalogued values. Observed Hαextinction values are determined using observed Hα and cataloguedradio fluxes. Hα extinction values are also derived fromcatalogued Hα and Hβ flux values by means of an RVdependent extinction law. RV is then calculated in terms ofobserved extinction values and catalogued Hα and Hβ fluxvalues. Comparing observed and catalogue extinction values for a subsetof bulge objects, observed values tend to be lower than catalogue valuescalculated with RV= 3.1. For the same subset we calculate= 2.0, confirming that toward the bulgeinterstellar extinction is steeper than RV= 3.1. For theinner Galaxy, a relation with the higher supernova rate is suggested,and that the low-density warm ionized medium is the site of theanomalous extinction. Low values of extinction are also derived usingdust models with a turnover radius of 0.08μm.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The AB Doradus Moving Group
From radio to X-ray wavelengths, AB Doradus has been an intensivelystudied star. We have identified ~30 nearby star systems, each with oneor more characteristics of youth, that are moving through space togetherwith AB Dor. This diverse set of ~50 million year old star systems isthe comoving, youthful group closest to Earth. The group's nucleus is aclustering of a dozen stars ~20 pc from Earth that includes AB Doritself. The AB Dor moving group joins the previously known and somewhatyounger and more distant Tucana/Horologium and TW Hydrae associationsand the β Pictoris moving group as excellent laboratories forinvestigations of forming planetary systems.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The distribution of G5-M stars in a region at the south galactic pole
A survey has been conducted of an 81-sq deg area at the south galacticpole (SGP) in order to study the late-type star distribution as afunction of distance from the galactic plane. Both spectral types andluminosity classes are obtained from objective-prism spectra, usingclassification criteria closely associated with the MK classificationsystem. The SGP data are compiled into a catalog of 2228 stars of typeG5 and later, to a limiting magnitude of V=13.5 mag. An analysis of thedensity distribution of both dwarfs and giants indicates that the numberof dwarfs, as a function of magnitude, appears inconsistent with theassumption of a uniform density distribution for these stars.
| Dwarf K and M stars in the southern hemisphere.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77..486U&db_key=AST
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| קבוצת-כוכבים: | לויתן |
| התרוממות ימנית: | 01h06m26.16s |
| סירוב: | -14°17'47.1" |
| גודל גלוי: | 9.453 |
| מרחק: | 49.975 פארסק |
| תנועה נכונה: | 102 |
| תנועה נכונה: | -93.8 |
| B-T magnitude: | 10.521 |
| V-T magnitude: | 9.542 |
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